The invention relates generally to clearance measurement systems, and more particularly to a clearance measurement system for measuring a clearance between a stationary component and a continuous rotary component of a rotating machine.
Various types of sensors have been used to measure the distance between two objects. In addition, these sensors have been used in various applications. For example, a steam turbine has a rotating bucket that is disposed adjacent a carrier. The clearance between the rotating bucket and the carrier varies due to various operating conditions, such as changes in temperature, oxidation of the bucket tip, and so forth. It is desirable that a gap or clearance between the rotating bucket and the carrier be maintained during operation of the steam turbine.
One existing sensor is a capacitance probe, which measures a capacitance for estimating the clearance between two components. Unfortunately, existing capacitance-based measurement techniques are limited in that they yield a direct current voltage based measurements for measuring clearances between stationary and rotating structures that are continuous in the direction of rotation. The measurements yield a static output in time, such as a direct current voltage level proportional to the clearance. As a result, the measurements do not account for changes in the clearance due to changes in temperature of the components, electronic drifts in the gain, offset of the electronics, oxidation of the bucket tip, and other factors.
Moreover, these clearance measurement systems are typically employed to measure clearances between components during design and offline testing. Unfortunately, these existing systems are ineffective for in-service measurements due to the noise and drift generated by changes in the geometry of the components, among other factors. Instead, in-service clearance control is based on the clearance measurements previously taken during design and offline testing of components. As the components become worn during service, the offline measurements become ineffective for in-service clearance control.
Accordingly, a need exists for providing a clearance measurement system that provides an accurate measurement of clearance between two components by minimizing the effect of calibration drift and noise in the system. It would also be advantageous to provide a self-calibrating clearance measurement system that could be employed for accurate clearance measurement for parts in operation.